In the presence of glucose the protein hexokinase 2 (Hxk2p), normally resident in the cytosol, is translocated to the nucleus where it impairs the activation of transcription of the glucose-repressed genes HXK1, GLK1 and SUC2, and promotes the activation of transcription of the glucose-induced genes HXK2 and HXT1. Here, we demonstrate the involvement of an heptameric motif, named the MED8 site, in the direct binding of the mediator protein Med8p, either as a monomer or as a homodimer. Because this site was previously involved in the Hxk2p-dependent glucose-induced regulation of gene transcription, we tested whether Hxk2p interacts with Med8p. Our results show that Hxk2 and Med8 proteins are physically associated and that this Hxk2p-Med8p interaction is of physiological significance because both proteins have been found interacting together in a cluster with DNA fragments containing the MED8 site. We conclude that Hxk2p operates through the MED8 site, by interacting with Med8p, in the glucose signal transduction pathway of Saccharomyces cerevisiae.
(c) 2002 Elsevier Science Ltd.